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Terraform

Terraform: ASG Not Updating on Launch Config Change

By Ondřej Dolanský on 12/30/2024

Learn how to force Terraform to update your auto-scaling groups when you change your launch configuration to avoid outdated instances.

Terraform: ASG Not Updating on Launch Config Change

Table of Contents

Introduction

When working with Terraform to manage your AWS infrastructure, you might encounter a situation where you need to update the launch configuration or launch template of your Auto Scaling group (ASG). However, simply modifying the ASG resource in your Terraform code won't automatically update the existing instances. This is because Terraform treats the ASG as a single entity and attempts to replace the entire group when changes are detected. To address this, there are several approaches you can take to ensure your instances are updated with the new configuration.

Step-by-Step Guide

When you update the launch configuration or launch template associated with your Auto Scaling group (ASG) in Terraform, it doesn't automatically update the existing instances in the ASG. Here's how to handle this:

  1. Understand the behavior: Terraform sees the ASG as a single resource. When you change its configuration, Terraform wants to replace the entire ASG, not just update the instances within it.

    resource "aws_autoscaling_group" "example" {
  # ... other configurations ...
  launch_configuration = aws_launch_configuration.example.name
}

  2. Use a new Launch Configuration/Template: The simplest approach is to create a new launch configuration or template with your changes and then update the ASG to use the new resource.

    resource "aws_launch_configuration" "example" {
  # ... your launch configuration ...
}

resource "aws_autoscaling_group" "example" {
  # ... other configurations ...
  launch_configuration = aws_launch_configuration.example.name 
}

  3. Force recreation of instances: You can trigger a replacement of instances by adding a lifecycle block with create_before_destroy to your ASG resource and making a dummy change to the ASG resource.

    resource "aws_autoscaling_group" "example" {
  # ... other configurations ...
  launch_configuration = aws_launch_configuration.example.name

  lifecycle {
    create_before_destroy = true
  }
}

  4. Rolling updates: For more controlled deployments, consider using the aws_autoscaling_policy resource to define update policies. This allows you to specify how many instances to update at a time, health check grace periods, and more.

    resource "aws_autoscaling_policy" "example" {
  # ... policy configurations ...
}

Remember to plan and apply your changes carefully to avoid unexpected downtime. Always test your updates in a staging environment before deploying to production.

Code Example

The code provides Terraform examples for updating instances within an Auto Scaling Group (ASG). It showcases three methods: creating a new launch configuration and referencing it in the ASG, forcing instance recreation by introducing a dummy change in the launch configuration, and utilizing an Auto Scaling policy for rolling updates. Each example includes explanations and emphasizes best practices like zero-downtime deployments and using a staging environment for testing. Remember to replace example values with your actual configuration.

This code demonstrates different ways to update instances in an Auto Scaling Group (ASG) when changing the launch configuration or template in Terraform.

1. Using a new Launch Configuration:

# Define a new launch configuration with the desired changes
resource "aws_launch_configuration" "example_new" {
  name_prefix                 = "example-lc-"
  image_id                    = "ami-0c55b159c41c1f27b"
  instance_type               = "t2.micro"
  security_groups              = [aws_security_group.example.id]

  # ... other launch configuration settings ...
}

# Define the Auto Scaling Group and reference the new launch configuration
resource "aws_autoscaling_group" "example" {
  name                       = "example-asg"
  max_size                    = 3
  min_size                    = 1
  desired_capacity            = 2
  launch_configuration        = aws_launch_configuration.example_new.name
  vpc_zone_identifier         = [aws_subnet.example.id]

  # ... other ASG settings ...

  # Optional lifecycle block to ensure new instances are created before old ones are terminated
  lifecycle {
    create_before_destroy = true
  }

  # Dummy tag to trigger update when launch configuration changes
  tags = {
    Name = "Example ASG"
    updated_at = timestamp()
  }
}

2. Forcing recreation of instances with a dummy change:

# Define the Auto Scaling Group with a lifecycle block
resource "aws_autoscaling_group" "example" {
  name                       = "example-asg"
  max_size                    = 3
  min_size                    = 1
  desired_capacity            = 2
  launch_configuration        = aws_launch_configuration.example.name
  vpc_zone_identifier         = [aws_subnet.example.id]

  # ... other ASG settings ...

  # Lifecycle block to ensure new instances are created before old ones are terminated
  lifecycle {
    create_before_destroy = true
  }

  # Dummy tag to trigger update when launch configuration changes
  tags = {
    Name = "Example ASG"
    updated_at = timestamp()
  }
}

# Make a dummy change to the launch configuration to trigger recreation
resource "aws_launch_configuration" "example" {
  name_prefix                 = "example-lc-"
  image_id                    = "ami-0c55b159c41c1f27b"
  instance_type               = "t2.micro"
  security_groups              = [aws_security_group.example.id]

  # ... other launch configuration settings ...

  # Dummy tag to force recreation
  user_data = "dummy-${timestamp()}"
}

3. Using an Auto Scaling Policy for rolling updates:

# Define the Auto Scaling Group
resource "aws_autoscaling_group" "example" {
  # ... ASG configuration ...
}

# Define an Auto Scaling update policy
resource "aws_autoscaling_policy" "example" {
  name                      = "example-policy"
  autoscaling_group_name = aws_autoscaling_group.example.name
  adjustment_type           = "ChangeInCapacity"
  scaling_adjustment        = 1
  cooldown                  = 300
  min_adjustment_step       = 1
}

Important notes:

  • Replace the example values with your actual configuration.
  • The create_before_destroy lifecycle setting ensures zero downtime during updates.
  • The dummy tag in the second example forces Terraform to recreate instances when the launch configuration changes.
  • The aws_autoscaling_policy resource provides more granular control over updates.
  • Always test your changes in a staging environment before deploying to production.

Additional Notes

  • Impact of Instance Replacement: Be aware that replacing instances will lead to a brief period where new instances are provisioned and added to the ASG, while old instances are terminated. This might cause temporary capacity fluctuations.
  • Alternatives to Dummy Changes: Instead of dummy tags, consider using the depends_on meta-argument in your ASG resource to explicitly define a dependency on the launch configuration. This can help Terraform recognize changes more reliably.
  • Blue/Green Deployments: For highly sensitive applications, explore blue/green deployments. This involves creating a new ASG with the updated configuration and gradually shifting traffic from the old ASG to the new one.
  • Terraform State: Terraform stores the state of your infrastructure. If you manually change resources outside of Terraform, you'll need to import those changes into your Terraform state to ensure consistency.
  • Security Groups and Load Balancers: When replacing instances, ensure your security groups and load balancer configurations are correctly associated with the new instances to avoid connectivity issues.
  • Monitoring and Logging: Implement robust monitoring and logging to track the update process, identify any potential issues, and ensure a smooth transition.
  • Terraform Best Practices: Adhere to Terraform best practices, such as using modules for reusability, managing remote state for collaboration, and leveraging workspaces for different environments.

Summary

This table summarizes how to update existing instances when changing the launch configuration or template of an Auto Scaling Group (ASG) in Terraform:

Method Description Terraform Example Pros Cons
New Launch Configuration/Template Create a new launch configuration/template with the desired changes and update the ASG to use it. terraform resource "aws_launch_configuration" "example" { # ... your launch configuration ... } resource "aws_autoscaling_group" "example" { # ... other configurations ... launch_configuration = aws_launch_configuration.example.name } Simple, avoids instance recreation. Requires managing multiple launch configurations/templates.
Force Instance Recreation Add a lifecycle block with create_before_destroy to the ASG resource and make a dummy change to trigger instance replacement. terraform resource "aws_autoscaling_group" "example" { # ... other configurations ... launch_configuration = aws_launch_configuration.example.name lifecycle { create_before_destroy = true } } Ensures all instances are updated. Can cause downtime depending on ASG configuration.
Rolling Updates Define update policies using the aws_autoscaling_policy resource for controlled deployments. terraform resource "aws_autoscaling_policy" "example" { # ... policy configurations ... } Provides fine-grained control over updates, minimizes downtime. More complex to configure.

Important:

  • Terraform treats the ASG as a single resource and attempts to replace it entirely when its configuration changes.
  • Always test your changes in a staging environment before deploying to production.
  • Plan and apply updates carefully to minimize downtime.

Conclusion

Managing updates to your Auto Scaling Groups in Terraform requires a nuanced understanding of how Terraform handles resource changes. Simply updating the launch configuration or template associated with your ASG won't automatically propagate those changes to existing instances. You can achieve the desired outcome by employing techniques like creating new launch configurations, forcing instance recreation with dummy changes, or implementing rolling updates using Auto Scaling policies. Each method comes with its own set of advantages and considerations, so choose the one that best suits your application's needs and your tolerance for downtime. Remember to always test your changes in a staging environment before deploying to production to ensure a smooth and error-free update process. By following these best practices and carefully considering the available options, you can effectively manage your ASG updates in Terraform while minimizing disruptions to your applications.

References

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